1. Field of the Invention
The present invention relates to improving end-to-end quality of service and increasing efficiency and throughput of in-call DTMF transport in long-delay telecommunication environments, particularly in geostationary mobile satellite wireless product systems.
2. Background and Description of Related Art
Dual Tone Multi-Frequency (DTMF) signals are often used independently of a signaling system and after a call has been established to provide end-user access to specialized services. As defined in ITU-T Q.23, the 16 DTMF signals, i.e., 0, 1, 2, . . . 9, A, B, C, D, *, and #, are generated by push-button telephones using a combination of four low group frequencies and four high group frequencies. For clarity, such applications of DTMF signals will be referred to as xe2x80x9cin-call DTMFxe2x80x9d with individual transmission of each DTMF signal.
In-call DTMF transport based on out-band procedures have been used in wireless systems such as Global System for Mobile (GSM) Communications as described in the GSM 03.14 specification to achieve robust signaling. In the GSM specification, separate DTMF_START and DTMF_STOP messages are transmitted from the handset to the network for each digit pressed on the key pad. Furthermore, a DTMF_STOP can only be transmitted after an acknowledgement is received from the network for DTMF_START message. For wireless systems with long delay, especially geostationary mobile satellite communication systems, use of this protocol is extremely inefficient as explained below.
Use of GSM 03.14 design implies that two DTMF digits are separated by greater than 2*(round-trip delay+processing delay in forward direction+processing delay in reverse direction). For the geostationary satellite system environment with round-trip delays of the order of 520-540 ms, this means that interdigit separation is greater than 1.1 second (excluding processing delays). Thus the resulting interdigit separation in such an environment may be as high as 1.5 to 2 seconds when processing delays and interleaving delays are included. As an extreme example, a single press of a radial key on the handset that requires 10 digits to be sent to the network will take 15 to 20 seconds to reach the network. From an end-user perspective this can be extremely annoying.
It is noted that there exists other out-band DTMF transport protocols to enable the transfer of DTMF signals between devices such as described in U.S. Pat. No. 5,835,574 to Lam for xe2x80x9cDual-Tone Multi-Frequency Signal Transfer Protocol,xe2x80x9d issued Nov. 10, 1998. However, this DTMF protocol is applicable during the call setup phase of the call and is not applicable for in-call DTMF transport which is the subject of this invention. In the above-mentioned patent, the DTMF receiving entity first sends a DTMF ready tone for a prescribed interval, after which the DTMF transmitting entity sends a packet with DTMF digits. For in-call DTMF transport, however, the DTMF receiving entity is not even aware of an ensuing DTMF signal and therefore the technique mentioned in the prior art mentioned above cannot be used.
It would be desirable to provide improved end-to-end quality of service for in-call DTMF signal transport in long-delay telecommunication environments.
Schemes are employed in GSM to allow for a single key press in one message, i.e., two messages per digit, which forces the gap between two successive digits, as seen by the DTMF detector in the network, to be more than twice the round-trip delay in a wireless system such as a mobile satellite system, where the delay can be as high as 1.5 seconds. In TDMA systems where interburst interleaving is used to provide additional robustness, the interdigit gap may be as high as 2 seconds. As discussed, this means that a user may have to wait approximately 15-20 seconds before a response is received from the service, for a 10-digit number that the user may press from the telephone memory, assuming there is no retransmission.
In GSM systems, the mobile-to-mobile call is treated as a concatenation of a mobile-originated call (MOC) and mobile-terminated call (MTC); there being no guarantee on transporting DTMF from Network to mobile, and thus the transport of DTMF in a mobile-to-mobile call is not a guaranteed service. In-band techniques, such as those used in GSM Full Rate systems, do not guarantee reliable delivery of the in-call DTMF signals in the Network-to-AT mobile direction. This is because DTMF signals are processed as regular speech, therefore, the system models, quantizes and transmits the DTMF signal in the same way as speech. Hence, the output of the voice decoder at the AT is subject to signal modeling and quantization distortion.
Briefly summarized, the present invention relates to a system and a methodology that will improve the end-user quality of service both in terms of response time and reliability for the transport of in-call DTMF signals in wireless systems, particularly in geostationary mobile satellite systems. The methodology encompasses three distinct techniques to provide acceptable end-to-end quality of service for DTMF. The first technique is applicable for transport of DTMF in the wireless subscriber to network direction, where DTMF digits are carried in the form of an out-band message. The central part of the new technique is to allow multiple key presses in the same message, thereby increasing efficiency and throughput in long-delay environment. The second technique utilizes the vocoder""s functionality to carry DTMF in-band, thereby reducing system complexity. The scheme makes the use of an integrated DTMF detector which can classify a given frame of signal into several classes so that the DTMF encoded packet can carry a unique pattern across the air-interface to the voice decoder at AT, which is capable of identifying the pattern. The third technique pertains to the use of a message based DTMF transport between two ATs on a separate logical channel with a unique Service Access point Identifier (SAPI), thereby providing guaranteed service for DTMF transport in an AT-AT call.